1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and particularly, to an in-plane switching mode LCD device and a fabrication method thereof by which aperture ratio and the reliability of image quality can be improved.
2. Description of the Related Art
An LCD device of twisted nematic mode, which is mainly used in flat panel display devices having high image quality and low power consumption, has a narrow viewing angle. The refractive anisotropy of liquid crystal molecules together with the vertical orientation of the liquid crystal molecules with respect to the substrate when voltage is applied to a twisted nematic mode LCD device causes a narrow viewing angle. In contrast, an in-plane switching mode LCD has a wide viewing angle since the liquid crystal molecules are oriented in a direction parallel to the substrate when voltage is applied to an in-plane switching mode LCD device.
FIG 1A is a plan view showing a pixel of a related art in-plane switching mode LCD. FIG. 1B is a cross-sectional view along line I-I′ in FIG. 1A. As shown in FIG. 1A, gate lines 1 and data lines 3 are respectively arranged in longitudinal and transverse directions on a transparent first substrate 10 to define a pixel. In an LCD device with a panel having pixels, n gate lines 1 and m data lines 3 are crossed to make a panel having n×m pixels.
In the pixel, a thin film transistor 9 is formed adjacent to an intersection where one of the gate lines 1 and one of the date lines 3 cross over each other. The thin film transistor 9 includes a gate electrode 1a, source electrode 2a and drain electrode 2b that are respectively connected to the gate line 1, to the data line 3 and to the pixel electrode 7. A gate insulating layer 8 is formed above the gate electrode 1a. A semiconductor layer 5 is formed above the gate insulating layer 8. The source electrode 2a and drain electrode 2b are respectively formed in contact with an end of the semiconductor layer 5.
As shown in FIG. 1A, a common line 4 traverse across the pixel and is in parallel with the gate line 1. A pixel electrode line 14 overlaps the common line 4 as the common line 4 traverse across the pixel. Common electrodes 6, which branch from the common line 4, and pixel electrodes 7, which branch from the pixel electrode line 14, are arranged to be in parallel with each other for switching the liquid crystal molecules. The common electrodes 6 are formed simultaneously with the gate electrode 1a. The pixel electrode 7 is formed simultaneously with the both source electrode 2a and drain electrode 2b such that the pixel electrode 7 is connected to the drain electrode 2b of the thin film transistor 9. In addition, a passivation layer 11 and then a first alignment layer 12a are formed over the first substrate 10 including the source/drain electrodes 2a and 2b. 
As shown in FIG. 1B, the common electrodes 6 are formed adjacent to the periphery of the pixel to shield the pixel electrode from a lateral electric field generated by the data lines 3 formed on the periphery of the pixel. The pixel electrode line 14 overlaps the common line 4 with a gate insulating layer 8 therebetween to form a storage capacitor. A black matrix 21 for preventing light from leaking between pixels is formed on the surface of a second substrate 20 facing the first substrate 10. The black matrix 21 covers the thin film transistor 9, the gate lines 1, the data lines 3, and the common electrodes 6 adjacent to the data lines 3 on the first substrate 10. In addition, a color filter 23 with a second alignment layer 12b thereon is also formed on the surface of a second substrate 20 facing the first substrate 10. A liquid crystal layer 13 is formed between the first substrate 10 and the second substrate 20.
When a voltage is not applied to an in-plane switching mode LCD device as shown in FIGS. 1A and 1B, the liquid crystal molecules in the liquid crystal layer 13 are oriented corresponding to the alignment direction of the first alignment layer 12a and the second alignment layer 12b. However, when a voltage is applied between the common electrodes 6 and the pixel electrodes 7, the liquid crystal molecules are reoriented to be in parallel with the substrate and vertical to the extended direction of the common electrodes 6 and the data lines 3.
The liquid crystal molecules in the liquid crystal layer 13 is always reoriented on the same plane. Accordingly, gray level inversion does not appear to be generated when the panel of the LCD device is viewed from above, below, left or right of the LCD panel at an off angle to normal of the LCD panel. However, in the in-plane switching mode LCD device having the structure shown in FIGS. 1A and 1B, in which the black matrix 21 of the second substrate 20 covers the thin film transistor 9, the gate lines 1, the data lines 3, and the common electrodes 6 adjacent to the data lines 3 on the first substrate 10, light leakage may be generated due to a misalignment between the first substrate 10 and the second substrate 20. Thus, the width of the black matrix 21 has to be formed wider than the widths of the thin film transistor 9, the gate lines 1 and the data lines 3 to maintain misalignment error margin. As the width of the black matrix 21 is increased, the aperture ratio of the unit pixel is reduced. A decrease in the aperture ratio can reduce the resolution and brightness of the image displayed on the LCD panel of an LCD device.